Process for the manufacture of biodegradable starch esters

ABSTRACT

The invention covers a process to manufacture biodegradable starch esters which can be processed into extrudates, formed parts and sheeting. The starch is activated with a carboxylic acid/carboxylic anhydride mixture through partial swelling and the onsetting esterification reaction at simultaneous conversion of the water entrained in the starch and partially esterified with a carboxylic anhydride up to the desired degree of substitution with progressing swelling and disintegration.

[0001] This invention covers a process to manufacture biodegradablestrach esters which can be processed into extrudates, formed parts andsheeting as well as to cast films and deep drawing dies by means ofappropriate plasticizers.

[0002] A process to manufacture cellulose and starch esters, and morespecifically acetyl esters, has been known for some time (P.Schützenberger, C. R. hebd. Séances Acad. Sci. 61 (1865), 485-486; P.Schützenberger, C. R. hebd. Séances Acad. Sci. 68 (1869), 814-818).

[0003] The common process of making cellulose acetates by mineral-acidcatalytic esterification of acetic acid/acetic anhydride mixturesaccording to EP 0 146 936 cannot be easily transferred to themanufacture of starch esters since starches in a mineral acidic agentare subject to rapid hydrolytic decomposition.

[0004] Hence, neither the acidic catalytic esterification in an aqueoussolution as described in EP 0 204 353 nor the mineral acid additionstaggered over time as described in U.S. Pat. No. 205,863 are suited toproduce thermoplastic materials of sufficient strength and goodworkability.

[0005] By contrast, slow esterification of starches in amine solventssuch as pyridine according to U.S. Pat. No. 2,627,516 allows for analmost complete esterification of the hydroxyl groups of the starchwithout any essential hydrolytic chain degradation even in the presenceof strongly acidic reaction components according to EP 0 342 599.However, a problem here is the treatment of the esters since evenslightest amine residues will result in considerable odour problems andhealth impact during further processing stages. Also, it is extremelydifficult to recover the solvent.

[0006] For the above reasons, a number of tests were carried out toesterify starches without amine solvents or catalysts in non-aqueoussolutions.

[0007] Satisfactory results were only achieved with activated starcheswhereby the intramolecular and intermolecular hydrogen bridge bonds werelargely destroyed through swelling reactions in the acqueous phase andsubsequent water extraction (J. Muetgeert et al., Stärke 12 (10) 1958,303-308), whereas native starches are esterified after a very longreaction time only (J. Tranquair J. Soc. Chem. Ind. 28 (1909), 288 ff orH. T. Clarke; H. B. Gillespie J. Amer. Chem. Soc. 54 (1932), 2083-88).

[0008] A clearly more straightforward process for slow alkalinecatalytic esterification of starches is reported by MARK and MEHLTRETTER(Stärke 24, (3), pp. 73-76 (1972). The starch catalyst ratio determinedthrough systematic trial and error is found again in several laterpublications.

[0009] The disadvantage of this process, however, is that a considerableexcess of acid anhydride is required to achieve short reaction times,and hence a defined adjustment of a desired degree of substitutionthrough shortening the reaction time is hardly reproducible.

[0010] The esterification processes described in DE 4 114 185 are animprovement based on the process described by MARK and MEHLTRETTER inU.S. Pat. No. 3,795,670, but here, too, considerable excess of acidanhydride is required, and the degree of subsitution can only beadjusted via the reaction time.

[0011] The process described in DE 4 223 471 allows a goodreproducibility of the desired degree of substitution and a considerablereduction of the acid anhydride excess. The alkaline-activated starch isflocculated from the aqueous phase, dried and esterified in a separatereaction step to yield translucent to yellowish starch esters of verygood mechanical properties.

[0012] The disadvantage of this process is that it is a costly andenergy-intensive two-stage process with intermediate flocculation anddrying.

[0013] The use of anhydrous acetic acid/acetic anhydride mixtures asacetylating agents is known as well. For example, C. A. Burkhard (RayonText. Month., 23 (1942), 340 ff) was able to make a starch triacetateafter a ca. 40-hour reaction at return temperature. Hence, theadequately efficient esterification of native, non-activated starches inacetic acid/carboxylic acid anhydride mixtures is only possible in thepresence of adequate acidic or amine/alkaline catalysts.

[0014] Esterification of activated starches is much faster and moreefficient. A fairly homogeneous distribution of substituents can beachieved at degrees of substitution as low as DS≈2. However, theconsiderable costs required for starch activation are in contrast tocommercialisation.

[0015] It is the intention of this invention to avoid the disadvantagesof the aforementioned state of engineering and to propose an economicprocess to make starch esters with defined adjustable degrees ofsubstitution at a largely homogeneous distribution of substituents.

[0016] This intention is met through claims 1 through 9 whereby claims 2through 6 are preferred embodiments of the invention.

[0017] Surprisingly, it was found that native or slightly chemicallymodified starches with degrees of substitution between 0.001 and 1 canbe activated with aliphatic monocarboxylic acids of a chain length of C₁to C₁₂ whereby the water from the starch or starch derivates isconverted with carboxylic anhydrides in a parallel quick reaction sothat there is only a very weak esterification reaction at the starchmatrix besides the conversion of moisture, and the carboxylic anhydridevolume necessary for a proper esterification reaction is only meteredwith progressing swelling and destructuring of the starch.

[0018] A similar activation only with carboxylic anhydride was notfeasible. The starch esters made according to the process of theinvention are white, pourable powders which almost completely dissolvein solvents such as chloroform, acetone, ethylmethyl ketone, ethylacetate, and a definite turbidity only occurs at degrees of substitutionsmaller than DS≈2.0. At degrees of substitution between 1.8 and 2.7, thestarch esters made according to the invention can be processed intobiodegradable, translucent, brilliant formed parts, sheeting orthermoformed products with good mechanical properties with or withoutthe use of plasticizers.

EXAMPLE 1

[0019] 800 g of wheat starch (moisture content 13%)

[0020] 600 g of glacial acetic acid

[0021] 800 g of acetic anhydride

[0022] are heated to ca. 120 C. in a 10-liter agitator reactor with areflux condenser and maintained at this temperature for 30 minutes.Then, 1,100 g of acetic anhydride and 1,400 g of glacial acetic acid areadded over ca. 60 minutes and then maintained at ca. 125 C. for 6 hrs.The reaction mixture is cooled down to 90 C. and diluted with 3 kg ofglacial acetic acid, and flocculated in water. The flocculated wheatstarch diacetate is washed three to five times with water, and dried.

[0023] Yield: 1,000 g

[0024] DS: 2.30

EXAMPLE 2

[0025] 1,380 g of maize starch (moisture content 12.6%)

[0026] 500 g of acetic acid

[0027] 1,100 g of acetic anhydride

[0028] are heated to about 50 C. in a 10-liter agitator reactor withreflux condenser. Then, 1,700 g of acetic anhydride and 1,500 g ofglacial acetic acid are quickly added and maintained at ca. 125 C. atthe reflux for 3 hrs. After cooling to 90 C., the reaction mixture isdiluted with 3 kg of glacial acetic acid, and flocculated in water. Theflocculated maize starch diacetate is washed three to five times withwater, and dried.

[0029] Yield: 1,850 g

[0030] DS: 2.30

EXAMPLE 3

[0031] 1,380 g of high-amylose maize starch (moisture content 12%)-70%amylose

[0032] 300 g of glacial acetic acid

[0033] 950 g of acetic anhydride

[0034] are heated to 120 C. in a 10-liter agitator reactor with refluxcondenser and maintained at this temperature for 30 minutes. Then, 1,900g of acetic anhydride and 1,000 g of glacial acetic acid are added overca. 15 mins. and kept at the reflux at ca. 125 C. for 5 hrs. Aftercooling to 90 C., the reaction mixture is diluted with 2 kg of glacialacetic acid, and flocculated in water. The flocculated high-amylosemaize starch diacetate is washed three to fives times with water, anddried.

[0035] Yield: 1,900 g

[0036] DS: 2.31

EXAMPLE 4

[0037] 1,380 g of high-amylose maize starch (moisture content 12%)-85%amylose

[0038] 500 g of glacial acetic acid

[0039] 1,000 g of acetic anhydride

[0040] are heated to about 50 C. in a 10-liter agitator reactor withreflux condenser. Then, 1,900 g of acetic anhydride and 1,000 g ofglacial acetic acid are added over ca. 30 mins, and kept at ca. 125 C.at the reflux for 7 hrs. After cooling to 90 C., the reaction mixture isdiluted with 2 kg of glacial acetic acid, and flocculated in water. Theflocculated high-amylose maize starch diacetate is washed three to fivestimes with water, and dried.

[0041] Yield: 1,900 g

[0042] DS: 2.30

EXAMPLE 5

[0043] 750 g of potato starch (moisture content 18%)

[0044] 1,000 g of glacial acetic acid

[0045] 1,000 g of acetic anhydride

[0046] are heated to about 120 C. in a 10-liter agitator reactor andmaintained at this temperature for 15 mins. Then, 400 g of aceticanhydride are quickly added, and the reaction mixture is maintained atthe reflux at about 128 C. for 1 hour. After adding a mixture of 300 gof acetic anhydride and 2,500 g of glacial acetic acid, the reactionmixture is maintained at the reflux at ca. 123 C. for 5 hours. Aftercooling to 90 C., the reaction mixture is diluted with 2.5 kg of glacialacetic acid, and flocculated in water. The flocculated potato starchdiacetate is washed three to five times with water, and dried.

[0047] Yield: 850 g

[0048] DS: 1.8

EXAMPLE 6

[0049] 1,380 g of high-amylose pea starch (moisture content 13%)-80%amylose

[0050] 750 g of glacial acetic acid

[0051] 1,050 g of acetic anhydride

[0052] are heated to about 120 C. in a 10-liter agitator reactor andmaintained at this temperature for 60 minutes. Then, 1,000 g of aceticanhydride are quickly added, and the reaction mixture is maintained atthe reflux at about 128 C. for 2 hours. After adding a mixture of 1,100g of acetic anhydride and 2,000 g of acetic acid, the reaction mixtureis maintained at the reflux at ca. 125 C. for 8 hours. After cooling to90 C., the reaction mixture is diluted with 3 kg of glacial acetic acidand flocculated in water. The flocculated pea starch acetate is washedthree to fives times with water, and dried.

[0053] Yield: 1,950 g

[0054] DS: 2.6

EXAMPLE 7

[0055] 1,380 g of maize starch (moisture content 12.6%)

[0056] 300 g of glacial acetic acid

[0057] 500 g of acetic anhydride

[0058] are heated to about 120 C. in a 10-liter agitator reactor withreflux condenser. Then, 3,200 g of acetic anhydride are added uniformlyover 120 minutes whereby the temperature rises to ca. 130 C. Then, 1,500g of glacial acetic acid are added as quickly as possible, and thereaction mixture is kept at the reflux for 3 hours. After cooling to 90C., the reaction mixture is diluted with 3 kg of glacial acetic acid,and flocculated in water. The flocculated maize starch triacetate iswashed three to five times with water, and dried.

[0059] Yield: 2,080 g

[0060] DS: 2.92

EXAMPLE 8

[0061] 800 g of high-amylose maize starch (moisture content 12%)-70%amylose

[0062] 3,000 g of glacial acetic acid

[0063] 1,200 g of acetic anhydride

[0064] are heated to about 120 C. in a 10-liter agitator reactor withreflux condenser and maintained at this temperature for 60 minutes.Then, 400 g of acetic anhydride are added and kept at the reflux at ca.123 C. for 4 hours. After cooling to 90 C., the reaction mixture isflocculated in water. The flocculated high-amylose maize starchdiacetate is washed three to five times with water, and dried.

[0065] Yield: 1,100 g

[0066] DS: 2.35

EXAMPLE 9

[0067] 900 g of hydroxypropylated maize starch (moisture content12%)-DS≈0.07

[0068] 250 g of glacial acetic acid

[0069] 650 g of acetic anhydride

[0070] are heated to about 120 C. in a 10-liter agitator reactor andmaintained at this temperature for 30 minutes. Then, 1,200 g ofoenanthic anhydride are quickly added, and after heating to about 135C., 750 g of acetic anhydride are added over ca. 60 minutes. Afteranother 60 minutes reaction time, 1,500 g of glacial acetic acid areadded and stirred at about 130 C. for four hours. After cooling to 90C., the reaction mixture is diluted with 2 kg of glacial acetic acid,and flocculated in water. The flocculated maize starch diester is washedthree to five times with water, and dried.

[0071] Yield: 1,500 g

[0072] Total DS: 2.30

EXAMPLE 10

[0073] 800 g of oxidated maize starch (moisture content 12%)-0.65%carboxyl

[0074] 650 g of propionic acid

[0075] 600 g of acetic anhydride

[0076] are heated to about 130 C. in a 10-liter agitator reactor withreflux condenser and maintained at this temperature for 15 minutes.Then, 900 g of lauric anhydride are quickly added and stirred for 60minutes after heating to about 135 C. Then, 800 g of acetic anhydrideare added over 120 minutes. After a reaction time of 60 minutes, 1,500 gof glacial acetic acid are added and stirred at about 130 C. for 120minutes. After cooling to 90 C., the reaction mixture is diluted with 2kg of glacial acetic acid, and flocculated in water. The resulting maizestarch diester suspension is centrifugated twice, washed with water, andstripped with steam at about 95 C. for 15 minutes. Drying takes placeafter the steam-stripped product has been water washed once again.

[0077] Yield: 1,200 g

[0078] Total DS: 2.30

1. A process to manufacture biodegradable starch esters, characterizedin that the starch a) is activated through partial swelling and anonsetting esterification reaction at simultaneous conversion of thewater entrained in the starch with a carboxylic acid/carboxylic acidanhydride mixture, b) is partially esterified with progressing swellingand destructuring with a carboxylic acid anhydride up to the desireddegree of substitution.
 2. The process according to claim 1 ,characterized in that the carboxylic acid/carboxylic acid anhydridemixture employed in step a) consists of 15 to 450%, preferably 20-100%carboxylic acid, relative to the starch volume employed, as well as of0.4-3 equivalents, preferably 0.5-1.5 equivalent carboxylic acidanhydride, relative to the water entrained in the reaction mixture. 3.The process according to the claims 1 and 2, characterized in that step1 b) is initiated after a reaction time of 1-100 minutes, preferably 5to 60 minutes, by adding 0.5 to 1.5 equivalents, preferably 0.8 to 1.3equivalents carboxylic acid anhydride relative to the desired degree ofsubstitution.
 4. The process according to the claims 1-3, characterizedin that step b) is continued for 30-720 minutes, preferably 120 to 600minutes, until achievement of a complete disintegration and the desireddegree of substitution of the partially esterified starch, wherein anadditional 0% to 500% carboxylic acid, relative to the starch volumeemployed, are added.
 5. The process according to the claims 1-4,characterized in that the reaction is performed in a temperature rangebetween 50° C.-180° C., preferably between 75° C.-145° C.
 6. The processaccording to the claims 1-5, characterized in that the reaction isperformed in a single-pot process without isolation of intermediateproducts.
 7. The process according to the claims 1-6, characterized inthat both native starches such as maize starch, wheat starch, potatostarch, rice starch, pea starch, wax maize starch, high-amylose maizestarch, pea starch, tapioca starch or potato starches as well as theirderivatives such as hydroxy ethylated, hydroxy propylated, acetalated,phosphorylated, sulfated, oxidated, carboxy alkylated, benzylated,nitrated allyl starches, xenthate starches, and carbamyl starches withdegrees of substitution of between 0.001-1, preferably 0.1-0.5 in anaturally moist or predried state are used as starch components.
 8. Theprocess according to the claims 1-7, characterized in that theanhydrides of aliphatic carboxylic acid with the chain lengths C₂ toC₂₂, preferably C₂ to C₁₂, or the cyclic anhydrides of the maleic acid,the malonic acid, the succinic acid, the glutaric acid as well as theirderivatives or mixtures of said carboxylic acid anhydrides are used asacid anhydride components.
 9. The process according to the claims 1-8,characterized in that aliphatic monocarboxylic acids with chain lengthsof C₁ to C₁₂, preferably C₂ to C₅, are used as carboxylic acids.